1. Field of the Invention
This invention relates generally to optically generating a pattern of figures such as arrays of image points, spots, or lines. More particularly, this invention relates to generating such patterns using a rotating component that has a plurality of axicon deflection segments.
2. Description of the Related Art
The optical generation of a pre-defined pattern of spots or image structures is used in a variety of applications. Digital copiers, hand-held bar code scanners, industrial cutting or welding operations, printers, fingerprint identification, light show entertainment, telecommunications switching, medical applications, and optical displays are a few examples. Among the most common mechanisms for generating patterns of figures are tilting mirrors (e.g. oscillating mirrors driven by galvanometers) and reflection from rotating convex polygons.
However, optical pattern generators based on tilting mirrors typically have characteristics that make them unsuitable for certain applications. For example, pattern generation in these devices is achieved by scanning the tilting mirror back and forth. But oscillating, or back-and-forth scanning, of a tilting mirror requires that the mirror movement come to a stop and then reverse its direction. This takes time, which limits the rate at which the pattern can be produced. To increase the pattern generation rate, the mirror in these systems is often driven near the galvanometer resonant frequency. However, this severely restricts the patterns that can be produced and reduces the system duty cycle. For example, it is difficult to produce irregular patterns because the mirror motion is restricted to be oscillatory. The near-resonance condition also limits the range of pattern generation rates that can be achieved. For example, it is difficult to tune such a system over a wide range of rates since the near-resonance condition only exists over a small range of rates. In addition, the angular velocity of resonant pattern generators is usually sinusoidal and not suitable for a large number of applications where dwell time at each point must be reasonably constant and reasonably long in duration.
If a two-dimensional image pattern is desired (e.g. a series of parallel lines or a two-dimensional pattern of spots), then typically a single mirror is tilted in two directions simultaneously or two coordinated tilting mirrors are used.
In many cases the efficiency or duty cycle of the energy source such as a laser is also important. The efficiency or duty cycle may be defined as the fraction of energy deposited in a desired pattern at the treatment location compared to the total energy delivered by the source in a given period of time. If a pattern is sparse compared to the background field of view, it is preferable to turn off the energy source and scan quickly over the unexposed background, and then turn the source back on when the image has settled over the image point to be exposed. This requires an even more responsive device that can accelerate, decelerate, and settle quickly. As a result of these requisite characteristics, galvanometer or convex polygon or other prior art mechanisms are not well suited for high-speed pattern generation, particularly if the pattern is an irregular or a sparse pattern.
In the rotating convex polygon systems, the multiple sides of a three-dimensional polygon are made reflective and the polygon is rotated about a central axis. As each reflective side of the polygon rotates through an incident optical beam, the optical beam is reflected to generate a locus of points on a scanned line. The rotation of each reflective side produces a different scanned line. If all of the polygon sides are identical then the same line is scanned by each of the polygon sides. If the reflective sides have different prism angle with respect to the central rotation axis then each side produces a different displaced scan line.
However, the rotating polygon approach also has drawbacks that make it unsuitable for certain applications. For example, systems that produce a series of scan lines can suffer from aberrations induced by the polygon rotation. In order to produce a series of scanned lines, each polygon side must have a different pyramidal angle that offsets the scan line in a direction perpendicular to the direction of scan. However, as each side rotates through the incident optical beam, the angular orientation of the reflective polygon side changes with rotation angle. This can cause changes in the amount of offset as a function of rotation angle and/or introduce other unwanted image aberrations. One example of an unwanted image aberration is line bow. The ideal scan line is usually a straight line, but the actual scan line is often a bow-shaped arc due to the change in reflective surface angular orientation with polygon rotation. The sag of the image arc is the “bow” in the scan line and the amount of bow is usually dependent upon the amount of pyramidal angle on the reflective side of the polygon. When different lines are scanned using different pyramidal angles on the polygon sides, different amounts of line bow are produced for each scanned line.
Scan line bow and other effects caused by polygon rotation can cause additional problems, depending on the application. For example, in some applications the scanning action is used to compensate for motion of the scanner system relative to the treatment location (or surface) so that the optical pattern ideally remains fixed on the treatment location as the optical system is moved with respect to the surface. In this case, scan line bow and other polygon-induced aberrations will cause the optical pattern to move in the direction perpendicular to the scan direction. The unwanted movement of the optical pattern with respect to the treatment location/surface results in an unwanted image blur.
Optical pattern generators such as the galvanometer-driven mirror or the convex rotating polygon typically require complex multi-element anamorphic optical systems to produce image points that have different dimensions in two orthogonal directions. Some applications require an image that is strongly elliptical rather than circular. Industrial welding applications and some medical applications require this condition of an anamorphic image structure. The complexity of these optical systems makes it very difficult to keep the same image structure geometry across the area of the treatment location because the anamorphic optical systems have aberrations that vary with image field location.
Therefore, there is a need for optical pattern generators that can operate at high speeds with high duty cycle, with possibly long dwell times at each image point, and particularly for the generation of irregular patterns. There is also a need for pattern generators with reduced aberrations and/or reduced image blurring. There is a further need in many applications for image structures that are anamorphic and relatively constant in shape across the image field.